"The Development of Roman Mailed Cavalry," by John W. Eadie, from Journal of Roman Studies, Vol. 57, No. 1/2 (1967), pp. 161-17, in 17 bookmarked and searchable pdf pages, with plates. The article begins with a historical account of armored cavalry in Assyrian days, then focuses on Rome from the 2nd century B.C. through the early-4th century A.D. Wikipedia has a group of entries relating to the topic.